There is a vehicle lighting device including a plurality of light emitting diodes (LEDs) which are connected in series.
Here, a voltage applied to the vehicle lighting device fluctuates. Therefore, in the vehicle lighting device, an operating voltage range (voltage fluctuation range) is determined.
In addition, the light emitting diode has a forward voltage drop. Therefore, when the voltage applied to the plurality of light emitting diodes connected in series decreases, an amount of light emitted from the plurality of light emitting diodes decreases and there is a concern that a total luminous flux of the vehicle lighting device becomes less than a specified value.
Therefore, a technique is proposed in which when the voltage applied to the vehicle lighting device decreases, a current does not flow through some light emitting diodes among the plurality of light emitting diodes which are connected in series.
Thus, even when the voltage applied to the vehicle lighting device decreases, it is possible to secure a required total luminous flux.
However, when the current does not flow through some light emitting diodes, there is a new problem that the current flowing through remaining light emitting diodes suddenly increases and the total luminous flux rapidly increases.
Therefore, it is desired to develop a technique in which even when the voltage applied to the vehicle lighting device decreases, it is possible to secure a required total luminous flux and to suppress fluctuation of a total luminous flux.